A 60-year-old man presents with sudden onset severe tearing chest pain radiating to his back. He is hypertensive, tachycardic, and diaphoretic. Blood pressure is 190/110 mm Hg in the right arm and 150/90 mm Hg in the left arm. ECG shows non-specific ST-T changes, and troponin is normal. A widened mediastinum is seen on chest radiograph. What is the most appropriate next step in management?